The mature immune system of animals differentiates between self-molecules and non-self-molecules and mounts an immune response only against the latter. The immune system learns which molecules are self through constant exposure to those molecules that are normally a part of the animal. Thus, the mature immune system is tolerized to the presence of molecules that are self. However, the immune system is not tolerized to molecules that are newly presented in the animal. These molecules can be antigens and thereby stimulate an immune response against them. Commonly, newly presented antigens are from an extracorporeal source, such as an infection. In this case, the immune response helps to destroy the source of the antigens and thereby clear the infection from the body.
Newly presented antigens are produced in vivo through the degradation of cellular components. When the immune system recognizes these degradation products of self molecules as “non-self” antigens, an immune response can be mounted against them and an autoimmune disease can develop. Thus, these antigens are members of the class of molecules generally referred to as autoantigens and the antibodies produced against them are referred to as autoantibodies. For clarity herein, autoantigen is used to refer to the complete self molecule as found in the body. Autoantigenic fragment is used to refer to the degradation product of the autoantigen. Thus it is when an epitope is presented to the immune system as autoantigenic fragment that an immune response is elicited. Once elicited, the immune response can target the autoantigenic fragment, the autoantigen, or both.
Autoimmune diseases are diseases in which a specific immune response to self-molecules occurs, often leading to tissue and organ damage and dysfunction. The diseases can be organ-specific (e.g. Type I diabetes mellitus, thyroiditis, myasthenia gravis, primary biliary cirrhosis) or systemic in nature (e.g. systemic lupus erythematosus, rheumatoid arthritis, polymyositis, dermatomyositis, Sjogrenís syndrome, scleroderma, and graft-vs.-host disease).
One source of autoantigenic fragments is cleavage of an autoantigen during apoptosis. Apoptosis is a morphologically and biochemically distinct form of cell death that occurs in many different cell types during a wide range of physiologic and pathologic circumstances (reviewed in (Jacobson et al., 1997; Thompson, 1995; White, 1996)). Studies report that specific proteolysis catalyzed by a novel family of cysteine proteases is of critical importance in mediating apoptosis (Chinnaiyan and Dixit, 1996a; Martin and Green, 1995; Thornberry and Molineaux, 1995). These proteases (termed caspases), cleave downstream substrates after a consensus tetrapeptide sequence ending with aspartic acid (P1). The caspases are synthesized as inactive precursors that require specific proteolytic cleavage after an aspartic acid residue for activation (reviewed in (Nicholson and Thornberry, 1997)).
Granzyme B, a serine protease found in the cytoplasmic granules of cytotoxic T lymphocytes (CTL) and natural killer (NK) cells, has a similar requirement to caspases, for aspartic acid in the substrate P1 position (Odake et al., 1991; Poe et al., 1991). Studies have reported that granzyme B plays an important role in inducing apoptotic nuclear changes in target cells during granule exocytosis induced cytotoxicity (Darmon et al., 1996; Heusel et al., 1994; Sarin et al., 1997; Shresta et al., 1995; Talanian et al., 1997).
Granzyme B is described as catalyzing the cleavage and activation of several caspases (Chinnaiyan et al., 1996b; Darmon et al., 1995; Duan et al., 1996; Fernandes-Alnemri et al., 1996; Gu et al., 1996; Martin et al., 1996; Muzio et al., 1996; Quan et al., 1996; Sarin et al., 1997; Song et al., 1996a; Srinivasula et al., 1996; Talanian et al., 1997; Wang et al., 1996). Granzyme B also initiates caspase-independent pathways which contribute to target cell death. However, while several candidates for these additional pathways exist, they remain largely undefined (Sarin et al., 1997; Talanian et al., 1997).
One candidate pathway is the direct proteolysis of death substrates by granzyme B, although efficient non-caspase cellular substrates for this protease have not yet been identified. Initial studies have indicated that the cleavage of PARP, U1-70 kDa and lamin B observed during granzyme B-induced cell death is catalyzed by caspases, rather than directly by granzyme B (Darmon et al., 1995; Martin et al., 1996; Talanian et al., 1997), but the effects of granzyme B on other caspase substrates in vitro and during granule-induced cytotoxicity have not been extensively studied.